Polymeric coating for the protection of objects

ABSTRACT

A protective polymeric coating is applied to the surface of various objects which are to be exposed to a harsh environment. The protective polymeric coating covers the exposed surface, where the polymeric coating includes a polyimide polymer. The polyimide polymer in the polymeric coating has a backbone with at least one non-terminal phenyl group. A linkage is connected to the non-terminal phenyl group, where the linkage can be an amide linkage or an ester linkage. An oligomeric silsesquioxane compound is connected to the linkage through an organic substituent, where the oligomeric silsesquioxane is not incorporated into the polymer backbone. The polymeric coating provides protection to the underlying object.

The present application is a continuation in part of, and claimspriority from, non provisional U.S. patent application Ser. No.12/619,641, filed Nov. 16, 2009, where U.S. patent application Ser. No.12/619,641 claims priority from non-provisional U.S. patent applicationSer. No. 11/972,768, (now U.S. Pat. No. 7,619,042) filed Jan. 11, 2008,where U.S. patent application Ser. No. 11/972,768 claims priority fromprovisional U.S. Patent Application 60/970,571, filed Sep. 7, 2007. Thecontent of U.S. patent application Ser. Nos. 12/619,641, 11/972,768 and60/970,571 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a protective cover used for harsh environments,such as in outer space. In particular, this invention relates topolymeric protective coatings which can be applied to a wide variety ofobjects.

2. Description Related Art

Polyimides are an important class of polymeric materials and are knownfor their superior performance characteristics. These characteristicsinclude high glass transition temperatures, good mechanical strength,high Young's modulus, good UV durability, and excellent thermalstability. Most polyimides are comprised of relatively rigid molecularstructures such as aromatic/cyclic moieties.

As a result of their favorable characteristics, polyimide compositionshave become widely used in many industries, including the aerospaceindustry, the electronics industry and the telecommunications industry.In the electronics industry, polyimide compositions are used inapplications such as forming protective and stress buffer coatings forsemiconductors, dielectric layers for multilayer integrated circuits andmulti-chip modules, high temperature solder masks, bonding layers formultilayer circuits, final passivating coatings on electronic devices,and the like. In addition, polyimide compositions may form dielectricfilms in electrical and electronic devices such as motors, capacitors,semiconductors, printed circuit boards and other structures. Polyimidecompositions may also serve as an interlayer dielectric in bothsemiconductors and thin film multichip modules. The low dielectricconstant, low stress, high modulus, and inherent ductility of polyimidecompositions make them well suited for these multiple layerapplications. Other uses for polyimide compositions include alignmentand/or dielectric layers for displays, and as a structural layer inmicromachining applications.

In the aerospace industry, polyimide compositions have been used foroptical applications as membrane reflectors. In application, a polyimidecomposition is secured by a metal (often aluminum, copper, or stainlesssteel) or composite (often graphite/epoxy or fiberglass) mounting ringthat secures the border of the polyimide compositions. Such opticalapplications may be used in space, where the polyimide compositions andthe mounting ring are subject to repeated and drastic heating andcooling cycles in orbit as the structure is exposed to alternatingperiods of sunlight and shade. Satellites can remain in orbit forextended periods, so the materials used should survive as long as thesatellite remains functional.

Polyimide polymers are subject to rapid degradation in a highlyoxidizing environment, such as an oxygen plasma or atomic oxygen [AO],as are most hydrocarbon- and halocarbon-based polymers. AO is present inlow earth orbit [LEO], so many spacecraft experience this highlyoxidizing environment. The interactions between the oxidizingenvironment and the polymer material can erode and reduce the thicknessof the polymer material. To protect from erosion, protective coatingsincluding metals, metal oxides, ceramics, glasses, and other inorganicmaterials can be applied as surface treatments to polyimides subjectedto the oxidizing environment.

While these coatings are effective at minimizing the oxidativedegradation of the underlying material, they often experience crackingfrom thermal and mechanical stresses, mechanical abrasion, and debrisimpact. After cracking, the protective face is compromised and theunderlying polymeric material can be degraded from additional exposureto the oxidizing environment. Therefore, the availability of polymerswhich are able to resist AO degradation is very desirable.

Oligomeric silsesquioxanes [OS] can be incorporated into a polyimidematrix to improve the durability of polyimides in these environments.Polyimides with incorporated OS demonstrate excellent resistance to AOdegradation prevalent in LEO environments. Polyimides with incorporatedOS provide additional benefits as well. Polyhedral OS are referred to bythe trademark POSS™, and are a common form of OS.

Solar panels are often used to generate electrical power for anaircraft, airship, spacecraft, or satellite. Many solar panels use asemiconductor, such as silicon, in a photovoltaic (PV) structure toconvert light from the sun or from other light sources into electricity.These solar panels are often protected with a glass cover on the facedirected towards the sun. The glass cover provides a protective barrierfor the PV components. This glass cover protects the PV components fromdegrading upon exposure to many things, including atmospheric moisture,oxygen, atomic oxygen, sulfur, contaminants, and many other sources ofdegradation. The PV components can be exposed and degraded duringassembly, ground handling, integration, and flight. The glass coverexhibits high optical clarity which allows the transmission of light tothe underlying components, where the light is converted to electricalenergy. A glass cover can be significantly thicker than a polymericcoating, and greater thickness tends to result in greater weight.

Rockets, aircraft, or other lift engines, and lighter than air gases areoften used to carry a cargo into space or other high altitudeenvironments. Much of the total mass of a space rocket is the fuel usedto propel the rocket into space, and the mass of any cargo carried intohigh altitude environments is limited. A “high altitude environment” isdefined to include altitudes above 20,000 feet, the stratosphericnear-space environment, and space. Cargo carried into high altitudeenvironments is typically limited to a certain size and weight to fitwithin the confines and capabilities of the vehicle carrying the cargo.The cost per unit weight of delivering a vehicle, cargo, and payloadinto high altitude service is high, so many efforts are made to reducethe total weight. Reductions in the total space occupied by the cargoare also beneficial. The various components of a vehicle, cargo, and/orpayload should be able to withstand the harsh high altitude environmentfor the expected life. Reducing the cargo weight can save costs, orallow for additional components to be included with the vehicle. Effortsto find new materials or combinations of materials which can withstandthe high altitude environment and which can reduce the weight and volumeof the vehicle, cargo, and/or payload continue. Materials which canwithstand high altitude environments may also be beneficial in otherenvironments, such as certain industrial applications.

SUMMARY OF THE INVENTION

A protective polymeric coating is applied to the surface of variousobjects which are to be exposed to a harsh environment. The protectivepolymeric coating covers the exposed surface, where the polymericcoating includes a polyimide polymer. The polyimide polymer in thepolymeric coating has a backbone with at least one non-terminal phenylgroup. A linkage is connected to the non-terminal phenyl group, wherethe linkage can be an amide linkage or an ester linkage. An oligomericsilsesquioxane compound is connected to the linkage through an organicsubstituent, where the oligomeric silsesquioxane is not incorporatedinto the polymer backbone. The polymeric coating provides protection tothe underlying object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the formation of an amic acid.

FIG. 2 depicts the formation of an imide bond from an amic acid.

FIG. 3 depicts an oligomeric silsesquioxane (OS) compound.

FIG. 4 depicts a polyhedral shaped OS compound.

FIG. 5 depicts 4-4′-[hexafluoroisopropylidene]diphthalic anhydride[6-FDA]

FIG. 6 depicts 4,4′-oxydiphthalic anhydride [ODPA]

FIG. 7 depicts 1,3-diaminobenzoic acid [DBA]

FIG. 8 depicts 4,4′-diaminobiphenyl-3,3′-dicarboxylic acid [DBDA].

FIG. 9 depicts para-phenylene diamine [p-PDA]

FIG. 10 depicts the formation of a tethering amide bond by the reactionof an amine with a carboxylic acid.

FIG. 11 depicts a section of a representative polyimide polymer with theOS connected with an amide.

FIG. 12 depicts a section of a representative polyimide polymer with theOS connected with an ester.

FIG. 13 depicts a non-terminal phenyl group of the polymer backbone,with an OS compound labeled X connected with an ester or amide linkage,labeled XX.

FIG. 14 depicts different structures that represent the symbol YY inFIG. 13, with ZZ representing a direct bond, —O—, —S—, —SO—, —SO₂—,—CH₂—, —CF₂—, —C(CH₃)₂—, —(CF₃)₂—, —(CH₂)_(n)—, —(CH₂CHCH₃O)_(n)—,—((CH₂)₄O)_(n)—, —(Si(CH₃)₂O)_(n)—, —SiH(CH₃)O)_(n)—, —SiH(C₆H₅)O)_(n)—,or —Si(C₆H₅)₂O)_(n)—.

FIG. 15 depicts possible structures which are represented by the “WW” inFIG. 14.

FIG. 16 depicts a perspective view of an object with a sectioned view ofa polymeric coating covering the object.

NOTE: The use of waved lines “

” indicates the molecule continues, but does not necessarily repeat. Theuse of square brackets “[” and/or “]” indicates that the structurerepeats beyond the bracket. The use of round brackets “(” and/or “)”indicates substructures within a repeat unit and does not indicate thesubstructure repeats beyond the round brackets. In this description, anatom AA shown connected to a phenyl group through a bond, instead of atthe angles representing carbon atoms, is meant to depict the atom AAconnects to any available carbon atom in the phenyl group, and not to aspecific carbon atom. Therefore, such a drawing does not specificallydenote an ortho, meta, or para positioning of the bond to the AA atom.

DETAILED DESCRIPTION Polyimide

Polyimides are a type of polymer with many desirable characteristics. Ingeneral, polyimide polymers include a nitrogen atom in the polymerbackbone, wherein the nitrogen atom is connected to two carbonylcarbons, such that the nitrogen atom is somewhat stabilized by theadjacent carbonyl groups. A carbonyl group includes a carbon, referredto as a carbonyl carbon, which is double bonded to an oxygen atom. Mostpolyimides are considered an AA-BB type polymer because two differentclasses of monomers are used to produce the polyimide polymer. One classof monomer is called an acid monomer, and is usually in the form of adianhydride. The other type of monomer is usually a diamine, or adiamino monomer. Polyimides may be synthesized by several methods. Inthe traditional two-step method of synthesizing aromatic polyimides, apolar aprotic solvent such as N-methylpyrrolidone (NMP) is used. First,the diamino monomer is dissolved in the solvent, and then a dianhydridemonomer is added to this solution. The diamine and the acid monomer aregenerally added in approximately a 1:1 molar stoichiometry.

Because one dianhydride monomer has two anhydride groups, differentdiamino monomers can react with each anhydride group so the dianhydridemonomer may become located between two different diamino monomers. Thediamine monomer contains two amine functional groups; therefore, afterone amine attaches to the first dianhydride monomer, the second amine isstill available to attach to another dianhydride monomer, which thenattaches to another diamine monomer, and so on. In this matter, thepolymer backbone is formed. The resulting polycondensation reactionforms a polyamic acid. The reaction of an anhydride with an amine toform an amic acid is depicted in FIG. 1. The high molecular weightpolyamic acid produced is soluble in the reaction solvent and, thus, thesolution may be cast into a film on a suitable substrate such as by flowcasting. The cast film can be heated to elevated temperatures in stagesto remove solvent and convert the amic acid groups to imides with acyclodehydration reaction, also called imidization. Alternatively, somepolyamic acids may be converted in solution to soluble polyimides byusing a chemical dehydrating agent, catalyst, and/or heat. Theconversion of an amic acid to an imide is shown in FIG. 2.

The polyimide polymer is usually formed from two different types ofmonomers, and it is possible to mix different varieties of each type ofmonomer. Therefore, one, two, or more dianhydride-type monomers can beincluded in the reaction vessel, as well as one, two or more diaminomonomers. The total molar quantity of dianhydride-type monomers is keptabout the same as the total molar quantity of diamino monomers. Becausemore than one type of diamine or dianhydride can be used, the exact formof each polymer chain can be varied to produce polyimides with desirableproperties.

For example, a single diamine monomer AA can be reacted with twodianhydride comonomers, B₁B₁ and B₂B₂, to form a polymer chain of thegeneral form of (AA-B₁B₁)_(x)-(AA-B₂B₂)_(y) in which x and y aredetermined by the relative incorporations of B₁B₁ and B₂B₂ into thepolymer backbone. Alternatively, diamine comonomers A₁A₁ and A₂A₂ can bereacted with a single dianhydride monomer BB to form a polymer chain ofthe general form of (A₁A₁-BB)_(x)-(A₂A₂-BB)_(y). Additionally, twodiamine comonomers A₁A₁ and A₂A₂ can be reacted with two dianhydridecomonomers B₁B₁ and B₂B₂ to form a polymer chain of the general form(A₁A₁-B₁B₁)_(w)-(A₁A₁-B₂B₂)_(x)-(A₂A₂-B₂B₂)_(y)-(A₂A₂-B₂B₂)_(z), wherew, x, y, and z are determined by the relative incorporation ofA₁A₁-B₁B₁, A₁A₁-B₂B₂, A₂A₂-B₁B₁, and A₂A₂-B₂B₂ into the polymerbackbone. Therefore, one or more diamine monomers can be polymerizedwith one or more dianhydrides, and the general form of the polymer isdetermined by varying the amount and types of monomers used.

The dianhydride is only one type of acid monomer used in the productionof AA-BB type polyimides. It is possible to used different acid monomersin place of the dianhydride. For example, a tetracarboxylic acid withfour acid functionalities, a tetraester, a diester acid, or atrimethylsilyl ester could be used in place of the dianhydride. In thisdescription, an acid monomer refers to either a dianhydride, atetraester, a diester acid, a tetracarboxylic acid, or a trimethylsilylester. The other monomer is usually a diamine, but can also be adiisocyanate. Polyimides can also be prepared from AB type monomers. Forexample, an aminodicarboxylic acid monomer can be polymerized to form anAB type polyimide.

The characteristics of the polyimide polymer are determined, at least inpart, by the monomers used in the preparation of the polymer. The properselection and ratio of monomers are used to provide the desired polymercharacteristics. For example, polyimides can be rendered soluble inorganic solvents by selecting the monomers that impart solubility intothe polyimide structure. It is possible to produce a soluble polyimidepolymer using some monomers that tend to form insoluble polymers if theuse of the insoluble monomers is balanced with the use of sufficientquantities of soluble monomers, or through the use of lower quantitiesof especially soluble monomers. The term especially soluble monomersrefers to monomers which impart more of the solubility characteristic toa polyimide polymer than most other monomers. Some soluble polyimidepolymers are soluble in relatively polar solvents, such asdimethylacetamide, dimethylformamide, dimethylsulfoxide,tetrahydrofuran, acetone, methyl ethyl ketone, methyl isobutyl ketone,and phenols, as well as less polar solvents, including chloroform, anddichloromethane. The solubility characteristics and concentrations ofthe selected monomers determine the solubility characteristics of theresultant polymer. For this description, a polymer is soluble if it canbe dissolved in a solvent to form at least a 1 percent solution ofpolymer in solvent, or more preferably a 5 percent solution, and mostpreferably a 10 percent or higher solution.

Most, but not all, of the monomers used to produce polyimide polymersinclude aromatic groups. These aromatic groups can be used to provide anattachment point on the polymer backbone for a tether. A tether refersto a chain including at least one carbon, oxygen, sulfur, phosphorous,or silicon atom that is used to connect the polymer backbone to anothercompound or sub-compound. Therefore, if the polymer backbone wereconnected through the para position on a phenyl group, wherein the paraposition refers to the number 1 and the number 4 carbons on the benzenering, the ortho and meta positions would be available to attach a tetherto this polymer backbone. The ortho position to the number 1 carbonrefers to the number 2 and number 6 carbons, whereas the meta positionto the number 1 carbon refers to the number 3 and number 5 carbons.

Many polyimide polymers are produced by preparing a polyamic acidpolymer in the reaction vessel. The polyamic acid is then formed into asheet or a film and subsequently processed with heat (often temperatureshigher than 250 degrees Celsius) or both heat and catalysts to convertthe polyamic acid to a polyimide. However, polyamic acids are moisturesensitive, and care must be taken to avoid the uptake of water into thepolymer solution. Additionally, polyamic acids exhibit self-imidizationin solution as they gradually convert to the polyimide structure. Theimidization reaction generally reduces the polymer solubility andproduces water as a by-product. The water produced can then react withthe remaining polyamic acid, thereby cleaving the polymer chain.Moreover, the polyamic acids can generally not be isolated as a stablepure polymer powder. As a result, polyamic acids have a limited shelflife.

Sometimes it is desirable to produce the materials for a polyimidepolymer film, but wait for a period of time before actually casting thefilm. For this purpose, it is possible to store either a solublepolyimide or a polyamic acid. Soluble polyimides have many desirableadvantages over polyamic acids for storage purposes. Soluble polyimidesare in general significantly more stable to hydrolysis than polyamicacids, so the polyimide can be stored in solution or it can be isolatedby a precipitation step and stored as a solid material for extendedperiods of time. If a polyamic acid is stored, it will gradually convertto the polyimide state and/or hydrolytically depolymerize. If the storedmaterial becomes hydrolytically depolyermized, it will exhibit areduction in solution viscosity, and if the stored material converts tothe polyimide state, it will become gel-like or a precipitated solid ifthe polyimide is not soluble in the reaction medium. This reducedviscosity solution may not exhibit sufficient viscosity to form adesired shape, and the gel-like or solid material cannot be formed to adesired shape. The gradual conversion of the polyamic acid to thepolyimide state generates water as a byproduct, and the water tends tocleave the remaining polyamic acid units. The cleaving of the remainingpolyamic acid units by the water is the hydrolytic depolymerizationreferred to above. Therefore, the production of soluble polyimides isdesirable if there will be a delay before the material is formed forfinal use.

Soluble polyimides have advantages over polyamic acids besides shelflife. Soluble polyimides can be processed into usable work pieceswithout subjecting them to the same degree of heating as is generallyrequired for polyamic acids. This allows soluble polyimides to beprocessed into more complex Shapes than polyamic acids, and to beprocessed with materials that are not durable to the 250 degree Celsiusminimum temperature typically required for imidizing polyamic acids. Toform a soluble polyimide into a desired film, the polyimide is dissolvedin a suitable solvent, formed into the film as desired, and then thesolvent is evaporated. The film solvent can be heated to expedite theevaporation of the solvent.

Oligomeric Silsesquioxane

OS compounds or groups are characterized by having the general formulaof [RSi]_(n)[O_(1.5)]_(n) wherein the R represents an organicsubstituent and the Si and the 0 represent the chemical symbols for theelements silicon and oxygen. R can be aliphatic or aromatic, andincludes a wide variety of organic compounds. The silicon atoms areconnected together through the oxygen atoms, with the R groups connectedto the silicon atoms, as seen in FIG. 3. These OS compounds have hybridorganic and inorganic properties. The Si—O groupings provide theinorganic properties, and the attached R groups provide the organicproperties. Frequently, these OS compounds exist in a cage form suchthat a polyhedron is created by the silicon and oxygen atoms, as shownin FIG. 4. When the OS compound is in the cage form or the polyhedralform, the R groups are exterior to the cage, with the Si atoms generallyforming corners of the cage.

Frequently, the OS compound will have an organic substituent which has afunctional group. These OS compounds can therefore have organicsubstituents with varying structures connected to the different Si atomswithin a single OS compound. A typical example would be a polyhedral OSrepresented by the formula [RSi]_((n-1)) [R′A]₁[O_(1.5)]_(n), wherein R′symbolizes an organic substituent with a functional group which can beused to connect the OS compound to a polymer backbone or some othermolecule. In this case, the A is used to represent an element. Thiselement is usually Si, but can also be other elements, includingaluminum (Al), boron (B), germanium (Ge), tin (Sn), titanium (Ti), andantimony (Sb). These different atoms incorporated into the OS compoundprovide different characteristics which will be imparted to the polymer.

Attaching an OS group to a polyimide polymer can affect manycharacteristics of the polymer, including oxidative stability,temperature stability, glass transition temperature, solubility,dielectric constant, tensile properties, thermomechanical properties,optical properties, and other properties. One significant characteristicimproved by incorporation of OS in a polyimide polymer is increasedresistance to degradation in oxidizing environments, such as oxygenplasma and AO, as discussed above. Oligomeric silsesquioxanes [OS] canbe incorporated into a polyimide matrix to improve the durability ofpolyimides in these environments. Therefore, polyimide polymers withincorporated OS are desirable.

When a hydrocarbon or halocarbon polyimide that includes OS is exposedto an oxygen plasma or AO, the organic substituent portions of the OSoxidize into volatile products while the inorganic portion forms apassivating silica layer on the exposed surface of the polyimidepolymer. This process is referred to as the glassification of thepolymer. The silica layer tends to protect the underlying polyimidematerial from further degradation by the oxidizing environment.Additionally, the silica layer absorbs at least a portion of theultraviolet [UV] light, especially the UV light with a wavelengthshorter than about 256 nm. Therefore, the silica layer also protects thepolymer film from radiation, because UV light is a form ofelectromagnetic radiation. Additionally, if the silica layer exhibitssufficient thickness, it reduces gas and water permeability through thepolyimide film. It is possible to deposit additional silica on apolyimide film to produce the thickness necessary to significantlyreduce gas and water permeability through the film.

OS has been blended with polymers to provide a degree of protectionherein described, but the amount of OS which can be blended with apolymer is limited. Typically, the amount of OS incorporated into thepolymer by blending methods is limited to a concentration where the OScompounds do not aggregate into domains large enough to scatter visiblelight. Incorporation of additional OS above this level typically resultsin a reduction in optical and/or mechanical properties. However, it hasbeen found that chemically reacting the OS with the polymer reduces theOS aggregation and provides more uniform distribution of the OS withinthe polymer. As such, more OS can typically be incorporated into apolymer matrix via covalent bonding than by simpler blending methods.This results in a polymer which is better able to withstand exposure tooxygen plasma, AO, and UV radiation.

It is possible to attach the OS groups to a polyimide polymer byreacting the OS with one of the monomers before polymerization. Inpractice, however, this method is difficult owing to the high cost of OSand number of reaction and purification steps needed to obtain a usablemonomer of sufficient purity. High monomer purity is required for theformation of sufficient molecular weight polyamic acids and polyimides.For example, others have described a method to incorporate a polyhedralOS into the polyimide backbone requiring four reaction steps. The fewerthe reaction and purification steps, the lower the cost and the greaterthe efficiency of the entire process, as a general rule. Therefore, amethod of producing a desired polymer with fewer reaction andpurification steps is desirable, because such a method would probablyreduce the cost and improve the overall efficiency of the process. Thecurrent disclosure describes a polyimide composition incorporating OSand a method of synthesizing that polymer that uses two reaction stepsand zero to two purification steps.

Selection of Monomers

The characteristics of the final polymer are largely determined by thechoice of monomers which are used to produce the polymer. Factors to beconsidered when selecting monomers include the characteristics of thefinal polymer, such as the solubility, thermal stability and the glasstransition temperature. Other factors to be considered include theexpense and availability of the monomers chosen. Commercially availablemonomers that are produced in large quantities generally decrease thecost of producing the polyimide polymer film since such monomers are ingeneral less expensive than monomers produced on a lab scale and pilotscale. Additionally, the use of commercially available monomers improvesthe overall reaction efficiency because additional reaction steps arenot required to produce a monomer which is incorporated into thepolymer. One advantage of the current invention is the preferredmonomers are generally produced in commercially available quantities,which can be greater than 10,000 kg per year.

One type of monomer used is referred to as the acid monomer, which canbe either the tetracarboxylic acid, tetraester, diester acid, atrimethylsilyl ester, or dianhydride. The use of the dianhydride ispreferred because it generally exhibits higher rates of reactivity withdiamines than tetrafunctional acids, diester acids, tetraesters, ortrimethylsilyl esters. Some characteristics to be considered whenselecting the dianhydride monomer include the solubility of the finalpolymer as well as commercial availability of the monomers.

Certain characteristics tend to improve the solubility of the polyimidepolymer. These characteristics include flexible spacers, so-calledkinked linkages, and bulky substituents. The flexible spacer is an atomcovalently bonded between two separate phenyl groups. The phenyl groupsare relatively ridged, so the flexible spacer allows for increasedmotion between separate phenyl groups. Alkyl linkages are not as stableas the phenyl groups, so the use of simple hydrocarbon alkyl groupsbetween phenyl groups can reduce the stability of the polymer. Thestability of the overall polymer can be improved if the linkage issaturated with fluorine instead of hydrogen. Also, the use of otheratoms, such as Oxygen or Silicon, can result in a more stable polymer.

The term kinked linkages refers to a meta connection on a phenyl group.This means the polymer backbone would be connected through a number 1and number 3 carbon on a phenyl group. The use of kinked linkages, ormeta linkages, in the polymer backbone tends to result in a highercoefficient of thermal expansion, as well as greater solubility.

Bulky substituents in the polymer also tend to increase the overallpolymer solubility. Bulky substituents are compounds which are large andtend to interfere with intramolecular and intermolecular chainassociation because of their size. The bulky substituents can beincluded between phenyl groups in the backbone, connected directly to aphenyl group, or they can be tethered to the polymer backbone. The bulkysubstituents tend to reduce the ability of adjacent polymer chains totightly associate. This tends to allow solvent molecules to enterbetween adjacent polymer chains, which increases the polymer solubility.

Urging phenyl groups in the backbone to align in different planes alsotends to increase the polymer solubility, and bulky substituents can beused for this purpose. If two phenyl groups in the backbone arerelatively close together, and each has a bulky substituent connected ortethered to it, those bulky substituents will sterically interfere witheach other and tend to urge one phenyl group into a plane that isperpendicular to the plane of the other phenyl group.

The flexible spacers can be saturated with larger components such asfluorine to improve the solubility of the resulting polyimide polymer.Other preferred atoms for separating the phenyl groups include oxygenand sulfur. The preferred dianhydride monomers of the current inventionare 6-FDA and ODPA, as seen in FIGS. 5 and 6, but other dianhydridemonomers may also be used.

The OS group is usually attached to the diamine monomer owing to thegreater availability of diamine architectures as compared todianhydrides. Therefore, one of the diamine monomers used should have afunctional group independent of the two amines, so when the two aminesare incorporated into the polymer backbone the functional group is stillavailable for a subsequent chemical reaction. This functional group isreferred to as an attachment point because this is the point where theOS group is attached to the polymer backbone. A wide variety ofattachment points can be used, but a carboxylic acid is preferred. Theattachment point is incorporated into the length of the polymerbackbone, not just at the ends or terminus of each chain, so that moreOS compounds can be attached to the polymer. Therefore, the preferredattachment point is a carboxylic acid connected by a single bond to aphenyl group, wherein the phenyl group is non-terminal and the phenylgroup is part of the polymer backbone.

The monomers DBA and DBDA, as shown in FIGS. 7 and 8, are utilizedheavily because of the presence of a carboxylic acid group, which serveas attachment points for the OS groups. Other diamino monomers without afree attachment point can also be used. One example of such a monomer isp-PDA, as seen in FIG. 9. The higher the concentration of diaminomonomers without an attachment point, the lower the concentration ofdiamino monomers with a free attachment point. Since the OS is primarilyincorporated into the polymer through the attachment point, the overallconcentration of OS compounds in the final polymer can be controlled byvarying the ratio of diamino monomers with and without a free attachmentpoint. Many other diamino monomers can be used, including but notlimited to 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF),1,3 bis(3-aminophenoxy)benzene (APB), 3,3-diaminodiphenyl sulfone(3,3-DDSO2), 4,4′-diaminodiphenylsulfone (4,4′-DDSO2), meta phenylenediamine (m-PDA), oxydianiline (ODA), the isomers of4,4′-Methylenebis(2-methylcyclohexylamine) (MBMCHA), the isomers of4,4′-Methylenebis(cyclohexylamine) (MBCHA), the isomers of1,4-cyclohexyldiamine (CHDA), 1,2-diaminoethane, 1,3-diaminopropane,1,4-diamonbutane, 1,5-diaminopentane, 1,6-diaminohexane, anddiamonodurene (DMDE).

Any remaining free carboxylic acid groups at the end or terminus of apolymer chain also serve as attachment points. These terminal carboxylicacid groups are generally connected to a terminal phenyl group.Attachment points connected to non-terminal phenyl groups are needed toincrease the ratio of OS incorporation into the final polymer. Anon-terminal phenyl group is a phenyl group in the polymer backbonewhich is between two imide bonds in the polymer backbone.

Oligomeric Silsesquioxane Considerations

Incorporation of the OS group into the polymer matrix is oftenbeneficial for the reasons previously described. Usually, the OS groupis incorporated in a polyhedral cage structure, and the polyhedral OS isattached to the polymer. In such instances, the OS group will have atleast one organic substituent with a functional group for attaching tothe polymer. It is also possible for the polyhedral OS to have two ormore functional groups, in which case it can be used as a monomer or asa crosslinking component. For example, a polyhedral OS group includingtwo amine functional groups could be incorporated into the backbone ofthe polymer as a diamine monomer. Alternatively, if one OS functionalgroup were to attach to a first polymer chain, the second OS functionalgroup could attach to a different polymer chain, so the two polymerchains were cross-linked. The polymer chains can also be crosslinked bythe inclusion of other organic substituents which will crosslink thepolymer chains.

The current invention considers the attachment of the OS groups to thebackbone by a tether such that these OS groups do not form part of thebackbone of the polymer. This provides advantages as compared toincorporating the OS groups in line with the polymer backbone. Oneadvantage of pendant attachment is that OS and polyhedral OS are rigid.When rigid OS is incorporated in line with the polymer backbone, itgenerally increases the material stiffness and ultimately limits theamount of polyhedral OS incorporated. By attaching the OS groups to thebackbone by a tether, the bulk polymer stiffness is less affected withincreasing levels of OS incorporation.

It is advantageous to use a tether when attaching an OS group to apolyimide polymer backbone. Steric factors favor the use of a tetherbecause the OS group is usually bulky and the tether provides anattachment point apart from the bulk of the OS. The tether needs to belong enough to allow the OS group to react with the polyimide backbone,but it should be short enough to restrict the OS from aggregating intolarge domains that reduce the mechanical and optical properties. Keepingthe OS group close to the attachment point provides for the OS beingrelatively evenly distributed throughout the polymer. Also, keeping theOS group close to the attachment point provides for a bulky constituentnear the polymer backbone, which tends to improve the solubility of thepolymer. Some desired properties of the tether itself include theprovision of a stable and secure attachment, as well as many degrees offreedom of movement for the OS group. The tether can include functionalgroups, such as amides or esters. A tether or chain about 5 atoms longprovides enough length to allow attachment while keeping the OS groupclose enough to the attachment point.

Preferably, the OS compound will include one organic substituent with afunctional group for attachment to the polymer backbone. This organicsubstituent becomes the tether connecting the polymer backbone to the OScompound. The preferred functional groups for this attachment are eitheran amine or an alcohol. The amine or alcohol on the OS compound reactswith a carboxylic acid on the polymer backbone to form an amide or esterconnection, with the amide reaction shown in FIG. 10. The amide or esterthus formed includes either a carbonyl carbon and a linking nitrogen, asseen in FIGS. 10 and 11, or a carbonyl carbon and a linking oxygen, asseen in FIG. 12. The carbonyl carbon is connected by a single bond to aphenyl group, and the phenyl group is part of the polymer backbone. Thelinking nitrogen or oxygen is connected by a single bond to the carbonylcarbon, and the linking nitrogen or oxygen is also connected to theorganic substituent tether for the OS compound.

By using the process described above, a polyimide polymer is formedwhich contains either an amide or ester linking a non-terminal phenylgroup to the OS compound. This section of the polymer is shown in FIG.13. The nitrogen in FIG. 13 is from the imide bond in the polymerbackbone, XX represents either an oxygen atom or a nitrogen atom with anattached hydrogen, depending on if the OS group is attached by formingan ester or an amide, respectively. The symbol X represents an OScompound, including a tether between the XX atom and the OS compound.FIG. 14 depicts different structures that represent the symbol YY inFIG. 13. These different structures depend on the diamino monomersselected for the polyimide polymer. The symbol WW in FIG. 14 representseither a hydrogen atom, a free carboxylic acid, or another amide orester linkage with an attached OS compound, as shown in FIG. 15, whereinXX again represents either nitrogen with an attached hydrogen or oxygen.The symbol ZZ in FIG. 14 represents a direct bond, —O—, —S—, —SO—,—SO₂—, —CH₂—, —CF₂—, —C(CH₃)₂—, —(CF₃)₂—, —(CH₂)_(n)—,—(CH₂CHCH₃O)_(n)—, —((CH₂)₄O)_(n)—, —(Si(CH₃)₂O)_(n)—, —SiH(CH₃)O)_(n)—,—SiH(C₆H₅)O)_(n)—, or —Si(C₆H₅)₂O)_(n)—. In these Figs., an atom shownconnected to a phenyl group through a bond, instead of at the hexagonangles representing carbon atoms, is meant to depict that atom connectedto any available carbon atom in the phenyl group, and not to a specificcarbon atom.

Process

The process for creating the final polymer should involve as fewreactions and as few isolations as possible to maximize the overallefficiency. Minimization of the number of vessels or pots which are usedduring the production process also tends to improve efficiency, becausethis tends to minimize the number of reactions and/or isolations of thepolymer.

The first step is forming the polyimide polymer backbone. Some of thebasic requirements for this polymer backbone are that it be soluble,that it include an attachment point, and that it has many of thedesirable characteristics typical of polyimide polymers. Typically, thediamino monomers will be dissolved in a solvent, such asdimethylacetamide [DMAc]. After the diamino monomers are completelydissolved, the dianhydride monomer is added to the vessel and allowed toreact for approximately 4 to 24 hours. The use of an end capping agent,such as a monoanhydride or a monoamine, is not preferred until after thepolymerization reaction is allowed to proceed to completion. At thatpoint, the addition of phthalic anhydride or other monoanhydrideend-capping agents can be used to react with remaining end group amines.Adding end capping agents during the polymerization reaction tends toshorten the polymer chains formed, which can reduce desirable mechanicalproperties of the resultant polymer. For example, adding end cappingagents during the polymerization reaction can result in a more brittlepolymer.

At this point the monomers have reacted together to form a polyamicacid. It is desired to convert the polyamic acid to a polyimide. Theconversion of the polyamic acid to the polyimide form is known asimidization, and is a condensation reaction which produces water, asseen in FIG. 2. Because water is a by-product of a condensationreaction, and reactions proceed to an equilibrium point, the removal ofwater from the reaction system pushes or drives the equilibrium furthertowards a complete reaction because the effective concentration of theby-product water is reduced. This is true for chemical reactionsgenerally, including condensation reactions.

The water can be removed from the reaction vessel chemically by the useof anhydrides, such as acetic anhydride, or other materials which willreact with the water and prevent it from affecting the imidization ofthe polyamic acid. Water can also be removed by evaporation. Oneimidization method involves the use of a catalyst to chemically convertthe polyamic acid to the polyimide form. A tertiary amine such aspyridine, triethyl amine, and/or beta-picolline is frequently used asthe catalyst. Another method previously discussed involves forming thepolyamic acid into a film which is subsequently heated. This willvaporize water as it is formed, and imidize the polymer.

A third imidization method involves removing the formed water viaazeotropic distillation. The polymer is heated in the presence of asmall amount of catalyst, such as isoquinoline, and in the presence ofan aqueous azeotroping agent, such as xylene, to affect the imidization.The method of azeotropic distillation involves heating the reactionvessel so that the azeotroping agent and the water distill from thereaction vessel as an azeotrope. After the azeotrope is vaporized andexits the reaction vessel, it is condensed and the liquid azeotropingagent and water are collected. If xylene, toluene, or some othercompound which is immiscible with water is used as the azeotropingagent, it is possible to separate this condensed azeotrope, split offthe water for disposal, and return the azeotroping agent back to thereacting vessel.

If water is removed from the reaction vessel as a vapor it is possibleto proceed with the addition of the OS group without isolating thepolyimide polymer formed. However, if the water is chemically removed,it is preferred to precipitate, filter, and possibly wash the polyimidebefore adding the OS group. If the polymer is precipitated, the filteredsolid polyimide is used as a feed stock in the next step. It should benoted that when reference is made to isolating a polyimide, it refers tothe isolation of a polyimide at one point in the entire productionprocess. For example, if a polyimide was formed from the basic monomers,then precipitated and filtered, and then washed several times, thiswould count as a single isolation and purification, even though severalwashes were performed. Two isolations would be separated by a reactionstep.

The OS group is connected to the polyimide back bone previously formed.One of the first steps is finding a suitable solvent to dissolve the OSgroup as well as the polyimide polymer. Some examples of suitablesolvents include: methylene chloride, chloroform, and possiblytetrahydrofuran (THF). One way to control the amount of OS that isincorporated into the polymer is by controlling the amount of monomerwith an attachment point which was included in the formation of thepolyimide polymer. Another way would be to limit the amount of OS added.If OS is added at a stoichiometric quantity lower than the availableattachment points, then there would be free attachment points remaining.These free attachment points could be used for cross linking with otherpolymer chains, or they could be used for other purposes. Preferably,the OS group is a monofunctional group with one functional group forattachment to the polymer back bone. Preferably this functional group isan amine or alcohol, which will combine with the preferred carboxylicacid that is available as the attachment point on the polymer back bone.

If the OS group includes two or more functional groups, differentpolymer chains can be cross linked through this OS group. For this tohappen, the OS group would be linked to two separate polymer chains. Anamine on the OS group can be reacted with the carboxylic acid on thepolymer chain to form an amide bond. This is a condensation reaction inwhich water is produced as a byproduct. In order to drive the reactionto completion it is desirable for the water to be removed from thereaction system. This can be facilitated with a dehydrating agent, suchas dicyclohexylcarbodiimide, followed by subsequent isolation of thepolyimide polymer. Additionally, this amide-forming reaction can befacilitated with an acid catalyst. If the OS compound contains analcohol for reacting with the carboxylic acid, water is still producedas a byproduct and removing the water will tend to drive the reaction tocompletion.

An alternate possibility is to remove the water via azeotropicdistillation from the reaction vessel. This can be done by adding or bycontinuing to use an azeotroping agent such as xylene or toluene, thenvaporizing the water, separating the water from the reaction vessel, anddiscarding the water after it has exited the reaction vessel. This issimilar to the process described above for the imidization reaction.

The current process includes up to two isolations of the polyimidepolymer. The first possible isolation is after the polymer imidizationreaction, and the second possible isolation is after the OS group hasbeen attached to the polyimide polymer. The azeotropic removal of waterin the vapor form during a condensation reaction eliminates the need fora subsequent isolation. Therefore, if vaporous water is azeotropicallyremoved during one of either the polymer imidization reaction or the OSattachment reaction, the number of isolations needed for the productionof the final OS containing polymer is reduced to one. If vaporous wateris removed after both of the above reactions, it is possible to producethe final product with no isolations.

By using a carboxylic acid as the attachment point on the polyimidepolymer and an amine or an alcohol as the functional group on the OS, itis possible to connect the OS group to the polymer back bone usinggentle reaction conditions. For example, the reaction conditions for acarboxylic acid/amine attachment do not require rigorous material purityor dryness, and/or could be completed at the relatively low temperatureof approximately 25 degrees Celsius. Similar conditions are possible ifan alcohol is reacted with the carboxylic acid attachment point.

It should be noted that the cost of the OS compounds can be high and sothe use of a very efficient means for connecting the OS group to thepolyimide polymer is desired. The carboxylic acid and amine groupconnection is very efficient in terms of OS attachment to the polyimide,and can be utilized to produce connection efficiencies in the range ofabout 73% to about 99%. After the OS group has been connected to thepolyimide polymer, the product can be stored for use at a later time orit can be used immediately to produce polymer films or other desiredpolymer articles.

Polymer Uses

The polyimide polymer produced as described above can be used forseveral specific purposes. One important characteristic to consider isthe color of the polymer. Electromagnetic radiation with wavelengthsbetween 400 nm and 750 nm is generally regarded as the visible spectrum,since the human eye can only perceive these wavelengths. Polyimidepolymers usually absorb the shorter wavelengths of light up to aspecific wavelength, which can be referred to as the 50% transmittancewavelength (50% T). Light with wavelengths longer than the 50%transmittance wavelength is generally not absorbed and passes throughthe polymer or is reflected by the polymer, up to a point in theinfra-red range, well outside of the visible range, where a polyimidepolymer may begin to absorb some electromagnetic radiation again. The50% T is the wavelength at which 50% of the electromagnetic radiation istransmitted by the polymer. The polymer will tend to transmit almost allthe electromagnetic radiation above the 50% T, and the polymer willabsorb almost all the electromagnetic radiation below the 50% T, with arapid transition between transmittance and adsorption at about the 50% Twavelength. If the 50% T can be shifted to a point below the visiblespectrum, the polymer will tend to be very clear and appear colorless,but if the 50% T is in or above the visible spectrum, the polymer willbe colored. Polyimides which appear colored tend to absorb moreultraviolet and visible radiant heat than those which do not appearcolored.

Generally, the factors that increase the solubility of a polymer alsotend to push the 50% T lower, and thus tend to reduce the color of apolymer. Therefore, the factors that tend to reduce color in a polymerinclude flexible spacers, kinked linkages, bulky substituents, andphenyl groups which are aligned in different planes. The currentinvention provides a polyimide polymer with very little color or even apolyimide which is colorless. The term “colorless” is defined to mean afilm that exhibits a 50% T below 400 nm, so that the film does notexhibit color perceptible to the human eye, so a human will not seecolor when looking at the film.

A polyimide polymer with low color or one which is colorless is usefulfor several applications. For example, if a polyimide is used as a coverin a multi layer insulation blanket on a satellite, the absence of colorminimizes the amount of electromagnetic radiation that is absorbed. Thisminimizes the heat absorbed when the polymer is exposed to directsunlight. Temperature variations for a satellite can be large, and aclear and colorless polyimide polymer, especially one that is resistantto AO degradation, provides an advantage.

Display panels need to be clear and colorless, so as not to affect thequality of the displayed image. The current invention is useful fordisplay panels. In addition to optical clarity, a display panel shouldhave low permeability to water and oxygen, a low coefficient of thermalexpansion, and should be stable at higher temperatures. Thermalstability at 200 degrees centigrade is desired, but stability at 250degrees centigrade is preferred, and stability at 300 degrees centigradeis more preferred. The surface of the current polymer can be glassifiedby exposing the surface to an oxygen plasma. This causes the OS groupsto degrade to a glass type substance, which tends to coat and protectthe surface of the polymer. This glass layer is thin enough that thelayer bends and flexes without breaking. The glassified layer protectsthe polymer, and if the glassification layer is thick enough it couldlower the permeability of the resultant polymer to water and oxygen.After the surface of the OS containing polymer is glassified, thepermeability to oxygen and water vapor tends to be reduced, so the needfor additional measures to lower this permeability are reduced oreliminated. One example of an additional measure which could be used tolower permeability is to thicken the glassification layer by depositingsilicon oxide on the surface of the film. The effects of any traces ofcolor in the polymer are minimized by supplying the polymer in a thinfilm, such as 1 mil thick, 1.5 mils thick, or even at least up to 3 milsthick. The current invention has been used to produce colorless films atleast 1.5 mils thick, and it is expected that even thicker films can beproduced that are colorless. Thicker films are more durable, but theyare also heavier and tend to have greater color effects, with theopposite being true for thinner films.

Polyimide films tend to be very strong, so they can be used asprotective covers. For example, sheets of polyimide film can be placedover solar panels to protect the panels from weather and other sourcesof damage. For a solar panel to operate properly, it has to absorbsunlight. Polyimide polymers with low color or with no color are usefulto protect solar panels and other items where a view of the protectedobject is desired.

EXAMPLES Example 1

To a clean, dry, 5 liter (l) reactor equipped with an overhead stirrer,thermometer, and rubber septa were added 428.26 grams (g) APB, 81.88 gDBA, and 5591.08 g DMAc. The reactor was sealed and purged with dryargon while the solution was stirred vigorously with the overheadstirrer until the regents dissolved. To this solution was added 898.76 g6FDA, and the resultant slurry was stirred for 24 hours, at which point13.78 g phthalic anhydride (PA) was added to the reaction vessel andallowed to react for 4 hours. 501.18 g pyridine and 646.84 g aceticanhydride were added to this solution. The solution was stirred for 24hours at room temperature, 24 hours at 70° C., then cooled to roomtemperature and precipitated in deionized water. The recovered polymer,which is referred to as SPC, was washed three times with water, anddried overnight in a vacuum oven at 100° centigrade (C) to yield 1374.81g dry SPC (98% yield).

To a clean, dry, 5 L reactor equipped with an overhead stirrer,thermometer, and rubber septa were added 900.00 g of SPC prepared asdescribed above and 5406.75 g dichloromethane (DCM). The reactor wassealed and purged with dry argon while the solution was stirred untilhomogeneous. 398.75 g aminopropylisobutyl polyhedral OS, 86.55 gDicyclohexyl carbodiimide (DCC), and 1802.25 g DCM, and 891.00 gdimethylacetamide (DMAc) were added to this solution. The reaction wasallowed to proceed for 24 hours at room temperature. The reactor wasthen cooled to 0° C. for three hours during which time the contentsbecame heterogeneous. The contents of the reactor were drained, andfiltered to remove the precipitate. The recovered polymer solution wasprecipitated into ethanol, recovered, and rinsed three times withdeionized water. The OS-containing polymer was then dried in a vacuumoven at 110° C. for 48 hours to yield 1148.30 g dry polymer (94% yield).

Example 2

To a clean, dry, 5 L reactor equipped with an overhead stirrer,thermometer, and rubber septa were added 1050.00 g of SPC prepared inExample 1 and 4000.00 g tetrahydrofuran (THF). The reactor was sealedand purged with dry argon while the solution was stirred untilhomogeneous. 403.18 g aminopropylisobutyl polyhedral OS, 87.51 g DCC,and 2090.00 g THF were added to this solution. The reaction was allowedto proceed for 24 hours at room temperature. The reactor was then cooledto 0° C. for three hours during which time the contents becomeheterogeneous. The contents of the reactor drained, and filtered toremove the precipitate. The recovered polymer solution was precipitatedinto deionized water, recovered, and rinsed three times with deionizedwater. The OS-containing polymer was then dried in a vacuum oven at 110°C. for 48 hours to yield 1207.90 g dry polymer (98% yield).

High Altitude Environments

High altitude environments can degrade many materials over a relativelyshort period of time. “High altitude environments” is defined to includealtitudes above 20,000 feet, the stratospheric near-space environment,and space. In these high altitude environments, there is less atmosphereto filter the UV light, or to filter the vacuum ultraviolet light (VUV).Direct exposure to the UV and VUV light emitted by the sun can be verydamaging for many materials, including most polymers. On Earth'ssurface, much of the UV and VUV are filtered by the atmosphere, whichmakes this form of radiation far less intense and damaging than in highaltitude environments. Atomic oxygen presents an additional hazard inspace. Atomic oxygen is present in low earth orbit, and atomic oxygencan corrode and degrade materials quickly. Satellites orbiting in lowearth orbits should have materials capable of withstanding exposure toatomic oxygen. The harsh nature of high altitude environments requiresspecial materials to protect the structural and surface materials of theaircraft and spacecraft present.

Atomic oxygen and UV radiation degrade most known hydrocarbon-based ororganic materials, including high strength fibers used in compositessuch as polyaramide fibers. Other materials degraded in high altitudeenvironments include polyamides, polyesters, polybenzoxazole, epoxies,natural rubbers, synthetic rubbers, silicones, polyurethanes, polyureas,polyacrylates, polymethacrylates, polyolefins, polycarbonates,cellulosics, polyethylenes, ultra high molecular weight polyethylene,carbon fibers, and many more. Many polyimides are damaged by atomicoxygen, but are resistant to damage from UV radiation. Some metals, suchas silver, are rapidly corroded by atomic oxygen. The thin ornon-existent atmosphere in high altitude environments does not providefor convective cooling. Consequently, the temperature of the structureis largely affected by radiative heat transfer via the absorption andemission of light and thermal radiation. Exposure to sunlight can resultin rapid heating, and periods of shade can result in rapid cooling. Thetemperature differentials for an object present in the high altitudeenvironment can be large, and the associated thermal expansion andthermal contraction can present design challenges for high altitudevehicles. Brittle materials can present extra challenges to avoid cracksand/or breaks.

The harsh nature of high altitude environments has limited the use ofmany materials on vehicles serving at these high altitudes. Certainmetals, such as aluminum, are used, but other materials which do notwithstand the high altitude environment are avoided or protected in somemanner before use. Different materials have different properties, andthe harsh environment can limit the selection of materials to performcertain tasks and/or functions. A carbon fiber boom used to support asolar panel array may have many desirable properties, such as highstrength, light weight, and low thermal expansion, but a differentmaterial which can withstand the harsh high altitude environment may beused despite properties which are less desirable.

Certain characteristics of high altitude environments can be found inindustrial applications. For example, UV light can be used to disinfectwater, and UV light is also used in protein analysis, DNA sequencing,and drug discovery. Oxygen plasmas can be used for certain etching andcutting processes, and oxygen plasmas include atomic oxygen. Theavailability of additional materials able to withstand these industrialconditions can be advantageous.

As discussed above, the cost per unit of weight to place something inhigh altitude service is high, so improvements which can reduce weightare desirable. There can also be limitations on the bulk of materialswhen carried to the high altitude service. Certain composites, such ascarbon fiber, can provide greater strength and less weight than certainmetals, but these composites are often not used because they degrade inthe high altitude environment.

Protective Coating for an Object

Aircraft and/or spacecraft are often assembled on the ground, and thenlaunched into service, typically from a runway, another aircraft, or arocket, where the term “aircraft” includes airplanes, airships, andother objects that travel through the atmosphere. The term “spacecraft”includes vehicles used above the earth's atmosphere. This subjects theaircraft or spacecraft to stress and vibration from the thrust of thelaunch. The cost of launching a vehicle, cargo, and payload into serviceis relatively high, so products which reduce weight are desired. Once inservice, the airplane, airship, satellite, or other spacecraft can beplaced in orbit, or it can be moved to various destinations as desired.

Many events can damage various components in a high altitudeenvironment. The various components may be re-arranged or re-positionedonce in place, such as when a satellite unfurls an array of solarpanels. Different components can be damaged during any re-positioningoperations, especially if the re-positioning does not go exactly asplanned. Many vehicles, once in service, have a relatively large surfacearea. The large surface area increases the chances of impact from amicrometeor, micrometeorite, or other debris. Stresses from the launchcan also damage a component or part. A component on an aircraft orspacecraft has many opportunities to be damaged, including stress fromthe launch, pre-launch damage, re-positioning issues, and impacts fromobjects once in service. Besides damages from impacts, launch stresses,and ground handling, vehicles in high altitude environments are exposedto relatively high levels of UV radiation, and may also be exposed toatomic oxygen.

A polymeric coating 10 can be applied to the surface 12 of an object 14for protection, as shown in FIG. 16, where the object 14 in FIG. 16 is abar. The polymeric coating 10 may be the outermost layer on the item orobject 14, so sunlight may strike the polymeric coating 10 beforepassing to any other materials or layers underneath. Any corrosivecompounds or other damaging items also contact the polymeric coating 10before contacting a surface 12 underneath the polymeric coating 10.Therefore, incident UV and VUV light must pass through the polymericcoating 10 before reaching the various components and layers underneath.Atomic oxygen also must pass through the polymeric coating 10 beforereaching the underlying object 14 if the polymeric coating 10 covers theentire surface 12 of the object 14.

The polymeric coating 10 also provides some protection from otherhazards, such as impacts, vibrational stress, and exposure to damagingcompounds. The polymeric coating 10 is somewhat flexible, so it may helpto absorb vibrations which can be present during launch. The polymericcoating 10 can also provide protection from materials that may beinadvertently applied to an objects surface 12, such as oils fromcontact with skin, material from spilled liquids, or paint overspray.The polymeric coating 10 also provides an external layer which iscontacted first when some foreign object contacts the object 14 beingprotected.

Many different materials can be protected with the polymeric coating 10,so design possibilities for objects 14 used in high altitudeenvironments may be increased. A material which is normally degraded byUV radiation or atomic oxygen can be protected by the polymeric coating10, so the effective lifetime of the object 14 in the high altitudeenvironment is increased. This increased effective lifetime can createopportunities to utilize different materials with desirable propertiesin high altitude environments. The different materials that can be usedbecause of the polymeric coating 10 can include such properties aslighter weight, increased strength, and reduced thermal expansion, aswell as a wide variety of other properties which may be desirable.Launch costs may be reduced, and new applications may become feasibledue to the use of different materials. Objects 14 which can be protectedinclude airplane wings, satellite bodies, satellite booms or struts,airship surfaces, satellite antenna, and any other surface which can beexposed to high altitude environments.

The polymeric coating 10 serves to protect the underlying materials ofthe object 14. The polyimide polymer described above is included in thepolymeric coating 10. Therefore, the method of protecting the object 14includes providing the object 14, and protecting the outer surface 12 ofthe object 14 with the polymer coating 10. This can include applying thepolymer coating 10 directly to the object 14. The polymer coating 10 canbe applied as a liquid and cured on the object 14, or it can be madeinto a film and either placed or adhered to the object surface 12, asdesired. Curing of the polymer coating 10 when applied as a liquid mayprimarily involve heating to evaporate the solvent, which can use alower temperature than heating to imidize a polyamic acid. Inalternative embodiments, the polymer coating 10 can be suspended nearthe object 14 or adhered to the object 14.

A polyimide polymer as described above is included in the polymericcoating 10. The polymeric coating 10 may contain other polymers,additives, or additional components as well, or the polymeric coating 10can be formed entirely from the polyimide polymer described above. Afilm formed from the neat polyimide polymer may be colorless, but thepolyimide polymer may be included as one component in a polymericcoating 10 where other components in the polymeric coating 10 arecolored. Therefore, the polymeric coating 10 may be colored even if afilm of the neat polyimide polymer would otherwise be colorless.Alternatively, the polymeric coating 10 may be formed entirely from thepolyimide polymer.

The polymer included in the polymeric coating 10 is produced by reactingat least one acid monomer with at least one diamine monomer to form apolyamic acid polymer backbone, where the monomers are selected suchthat the polyamic acid polymer backbone includes a non-terminalattachment point. Next, the polyamic acid polymer backbone can beimidized to produce the polyimide polymer. An oligomeric silsesquioxane(OS) compound can be attached to the attachment point on the polyimidepolymer backbone. The OS compound can be attached by reacting afunctional group connected to the OS compound with the attachment point.The attachment point can be a carboxylic acid, and the function groupcan be an amine, an alcohol, an epoxy, or other functional groups. Insome embodiments, the polymer backbone, in either the polyamic acid formor the polyimide form, is isolated a maximum of one time. Otherembodiments of polyimides with OS are also possible.

This provides a polymeric coating 10 that covers the object 14. Thepolymeric coating 10 includes a polyimide polymer with at least onenon-terminal phenyl group. The non terminal phenyl group is connected toa linkage, which can be an amide linkage formed by the reaction of thecarboxylic acid attachment point with an amine functional groupconnected to the OS. The linkage can also be an ester linkage formed bythe reaction of the carboxylic acid attachment point with an alcoholfunctional group connected to the OS. The polyimide polymer may alsocontain a mixture of amide and ester linkages by including OS with bothalcohol and amine functional groups, either simultaneously orsequentially.

The polymeric coating 10 has several desirable properties. The polymericcoating 10 is can be relatively thin, so an object 14 can be protectedwith a relatively small amount of polymeric material. Because smallamounts of material can be used, relatively little weight is added byusing the polymeric coating 10, which can help control weight for theobject 14. The polymeric coating 10 is flexible, so impacts, vibrations,or other stresses will not cause breaks or cracks as readily as someless flexible coating materials. In one embodiment, the polymericcoating 10 can be 1 mil thick, so it occupies very little space in thelaunch vehicle. The polymeric coating 10 can also be other thicknesses,as desired. The polymeric coating 10 transmits visible light and some UVlight, so the appearance of the object 14 is not significantly changed.Also, the polymeric coating 10 provides protection to the object 14 fromatomic oxygen and most UV and VUV light, as well as providing a topprotective layer for the object 14.

The polymeric coating 10 provides additional benefits as well. Thepolyimide is soluble, so it can be dissolved in a solvent and appliedonto the object 14 in solution. The polymeric coating 10 can then becured by simply evaporating the solvent, which can be at lowertemperatures than those necessary for imidizing a polyamic acid. Forexample, some solvents such as tetrahydrofuran, methylene chloride,chloroform, acetone, methyl ethyl ketone, methyl isobutyl ketone,monoglyme, diglyme, and many other solvents can be evaporated from apolymeric solution at a temperature not exceeding 150 degreescentigrade, so the polymeric coating 10 can be cured on the object 14 ata temperature not exceeding 150 degrees centigrade. A solvent can beevaporated at a temperature below the solvent boiling point, so lowtemperatures can be used to evaporate the solvent and cure the polymericcoating 10. Generally, lower temperatures require a longer period toevaporate solvent, so a lower cure temperature may increase the requiredcure time. Some objects 14 may be temperature sensitive, so reduced curetemperatures can prevent damage to the object 14. For example, apolymeric coating 10 can be cured at a temperature not exceeding 100degrees centigrade, or at a temperature not exceeding 50 degreescentigrade, or at a temperature not exceeding 23 degrees centigrade, asdesired. It is often possible to cure the polymeric coating 10 at a safetemperature for the object 14 by adjusting the cure time. Other factorscan also impact the cure time, such as air flow over the curingpolymeric coating 10 and the polymeric coating thickness.

The current invention has additional favorable properties. In someembodiments, the polyimide can be easily transported and stored as asolid for extended periods, which provides simpler logistics than for arelatively unstable polyamic acid. Oligomeric silsesquioxane isincorporated into the polyimide, so the polymeric coating 10 willexhibit self-healing upon exposure to atomic oxygen. The OS forms apassivating silica layer when exposed to atomic oxygen. If this layer isscratched or broken, the exposed polyimide below forms anotherpassivating silica layer when exposed to atomic oxygen. This helps tominimize harm caused by scratches or other damage to the polymer coating10. Additionally, the polyimide will not yellow as rapidly as many otherpolymers when exposed to UV, so the effective life of the object 14 maynot be shortened as much as with other polymers. Yellowing is a gradualincrease in the color of a polymer over time. References to a polyimidepolymer film being colorless are intended to describe the polyimide filmas produced, and before it has yellowed from use.

CONCLUSION

The polymeric coating 10 described has several desirable properties.These include relatively light weight, durability, relatively smallstorage space, ease of application, and protection of the object 14.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed here.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method of protecting an object comprising: (a)reacting at least one acid monomer with at least one diamine monomer toform a polyamic acid backbone, where the monomers are selected such thatthe polyamic acid backbone includes a non-terminal attachment point; (b)after step (a), imidizing the polyamic acid backbone to form a polyimidepolymer backbone; (c) after step (b), attaching an oligomericsilsesquioxane (OS) compound to the attachment point on the polyimidepolymer backbone, such that the OS compound is connected to thepolyimide polymer backbone; (d) providing an object with a surface; (e)protecting the surface from atomic oxygen and ultraviolet light with apolymeric coating, where the polymeric coating includes the polyimidepolymer backbone.
 2. The method of claim 1 where a polymer backbone isisolated a maximum of one time, where the polymer backbone includes thepolyamic acid backbone and the polyimide polymer backbone.
 3. The methodof claim 1 further comprising positioning the object in the presence ofatomic oxygen.
 4. The method of claim 1 where the monomers are selectedsuch that the polymeric coating is soluble.
 5. The method of claim 1where the entire surface is covered by the polymeric coating.
 6. Themethod of claim 1 where the OS is in the form of a polyhedron.
 7. Themethod of claim 1 where a functional group is connected to the OS andthe functional group is reacted with the attachment point to attach theOS to the polyimide polymer backbone, where the attachment point iscomprised of a carboxylic acid and where the functional group isselected from the group consisting of an amine and an alcohol.
 8. Themethod of claim 1 further comprising placing the object in a highaltitude environment.
 9. The method of claim 1 where the polymericcoating produces a layer of silicon dioxide when exposed to atomicoxygen.
 10. The method of claim 1 further comprising curing thepolymeric coating on the object at a temperature not exceeding 100degrees centigrade.
 11. The method of claim 1 further comprising curingthe polymeric coating on the surface of the object at a temperature notexceeding 150 degrees centigrade.
 12. An object comprising: a surface ofthe object; a polymeric coating covering the surface, where thepolymeric coating comprises a polyimide polymer, where the polyimidepolymer has a 50% transmittance wavelength shorter than the wavelengthsof the visible spectrum such that a film of the polyimide polymer iscolorless, the polymeric coating comprising: a polyimide polymerbackbone, where the polymer backbone includes at least one non-terminalphenyl group; a linkage selected from the group consisting of: an amidelinkage comprising a carbonyl carbon and a linking nitrogen atom,wherein the carbonyl carbon is connected by a single bond to thenon-terminal phenyl group in the polymer backbone and the linkingnitrogen atom is connected by a single bond to the carbonyl carbon; anester linkage comprising a carbonyl carbon and a linking oxygen atom,wherein the carbonyl carbon is connected by a single bond to thenon-terminal phenyl group in the polymer backbone and the linking oxygenatom is connected by a single bond to the carbonyl carbon; and acombination of the amide linkage and the ester linkage; at least oneoligomeric silsesquioxane (OS) compound comprising silicon atoms andoxygen atoms connected together, and an organic substituent connected toeach silicon atom of the OS compound, wherein at least one organicsubstituent is connected to the linkage.
 13. The object of claim 12where the object is a component of a space craft.
 14. The object ofclaim 12 where the object includes a carbon-based component.
 15. Theobject of claim 12 where the OS is in the form of a polyhedron.
 16. Theobject of claim 12 where the polymeric coating protects the object fromultraviolet light.
 17. The object of claim 12 where the polymericcoating has a thickness of 1.5 mils or less, and the polymeric coatingis colorless such that the human eye does not detect color in thepolymeric coating.
 18. An object resistant to atomic oxygen attackcomprising: an object including a surface; a polymeric coatingpositioned over the surface, where the polymeric coating comprises apolyimide polymer having a 50% transmittance wavelength shorter than thevisible spectrum such that the a film of the polyimide polymer iscolorless, the polyimide polymer having a polyimide polymer backboneincluding at least one unit represented by the following formula:

wherein X includes an oligomeric silsesquioxane (OS) compound, and XX isan oxygen atom; wherein YY is selected from the group consisting of thecompounds represented by the following formulas:

and wherein ZZ is selected from the group consisting of a direct bond,—O—, —S—, —SO—, —SO₂—, —CH₂—, —CF₂—, —C(CH₃)₂—, —(CF₃)₂—, —(CH₂)_(n)—,—(CH₂CHCH₃O)_(n)—, —((CH₂)₄O)_(n)—, —(Si(CH₃)₂O)_(n)—, —SiH(CH₃)O)_(n)—,—SiH(C₆H₅)O)_(n)—, and —Si(C₆H₅)₂O)_(n)—, and wherein WW is selectedfrom the group consisting of hydrogen and a compound represented by oneof the following formulas:


19. The object of claim 18, wherein the polymer includes at least oneunit represented by the following formula:


20. The object of claim 18 where the polymeric coating has a thicknessof 1.5 mils or less, and the polymeric coating is colorless such thatthe human eye does not detect color in the polymeric coating.